1. Field of the Invention
This invention relates to a magnetic powder composed of fine particles and having high magnetic anisotropy that is suitable for high density recording.
2. Background Art
The volume of information that needs to be recorded/stored has increased constantly in recent years owing to the application of digital and high-band technologies to sound/voice and video information. As the volume of stored information increases so does the magnitude of the damage/loss incurred should the information be lost. The importance of data backup in maintaining today's advanced information society can therefore be expected to increase still further in coming years. At the same time, the need for downsizing and boosting the capacity of the devices and media used for recording/storing such information is also continuing to intensify.
Among the different information recording technologies, magnetic recording has invariably maintained a central position owing to such features as nonvolatility, high recording density, read/write speed, and low cost per unit recording volume. Media such as magnetic tape and magnetic disks that are produced by applying a magnetic paint formulated to include magnetic particles to a film are especially superior to other media not only in productivity but especially in the low cost per unit recording volume and excellent removability realized by winding magnetic tape into rolls to achieve virtual three-dimensional recording. As these media are therefore anticipated to continue to play a critical role in data backup, strong market demand exists for products with improved recording density, reliability, durability and economy.
The recording capacity of magnetic recording media, e.g., hard disks, has been increasing at the rate of 60–100% annually in recent years. This advance is driven in part by utilization of MR (magneto-resistive) devices such as magnetic heads. The same MR device technology has also been applied to upgrade the recording density of magnetic tapes. The magnetic head employing an MR device is referred to as an “MR head.”
To obtain high reproduction output with a conventional magnetic induction head it is necessary to establish a large value of [residual magnetic flux density× magnetic layer thickness (hereinafter called Mr·t)]. In contrast, the output of the device used in an MR head, which has high sensitivity, saturates at an unnecessarily high Mr, so that increasing Mr·t has the reverse effect of decreasing sensitivity. It is also known regarding magnetic recording that noise decreases with decreasing Mr·t/Hc and increasing number of magnetic particles per unit volume of the medium. While the AMR head is currently the most widely used MR head, other heads with even higher sensitivity than the MR head, most notably the GMR head and the TMR head, are also under development. This further increase the need to reduce medium noise.
In the light of the foregoing, a magnetic powder capable of achieving high output and C/N ratio in the short wavelength region must meet the following criteria:    (1) At any given thickness of the magnetic layer, the coercive force Hc should be high and the saturated magnetization σs should be within an appropriate range at which the output does not saturate. In other words, it is important that σs be controllable without degrading other magnetic properties.    (2) The number of magnetic particles per unit volume should be great. The particles must be made fine to achieve a large number of particles per unit volume.
In view of these considerations, the techniques set out below, for instance, have been proposed for achieving high output and C/N in the short wavelength region.
JP 2001-6147A for example, teaches a ferromagnetic metal powder suitable for use in a high-density digital recording system whose properties include: major axis length of 30–120 nm, axial ratio of 3–8, Hc of 1,000–4,000 Oe, and σs of 100–180 emu/g.
JP 10-69629A filed by the present assignee teaches a magnetic powder for achieving superior magnetic properties of a high quality that is composed of Fe containing 5–50 at. % of Co, 0.1–30 at. % of Al, 0.1–10 at. % of rare earth elements (including Y), not more than 0.05 wt % of Periodic Table group 1a elements and not more than 0.1 wt % of Periodic Table group 2a elements and has Hc of 1,200–3,000 Oe and σs of 100–200 emu/g.
JP 2001-81506A filed by the present assignee teaches a precursor for producing a magnetic powder that is composed of acicular particles obtained by incorporating Co, Al, Si and R (where R represents at least one rare earth element, including Y) in iron oxy-hydroxide and that is used to coat the surface layer portion of particles containing Co at more than 0 to 50 at. % relative to Fe and further containing 0.1–30 at. % of dissolved Al with a layer containing 0.1–10 at. % of Si and 0.1–15 at. % of R. However, the powders taught by these publications are intended for a system requiring high output and are not intended for use in a system that utilizes one of the aforementioned types of MR heads (AMR head, GMR head or TMR head).
As regards σs, on the other hand, in contrast to iron, which has a σs of 214, magnetite and mag-hematite, two magnetic substances among the iron system oxides, have σs of 90 emu/g and 80 emu/g, respectively, and from the fact that hematite is a permanent magnet, metal magnetic particles produced from iron oxy-hydroxide or iron oxide as a precursor and finally obtained by reduction processing will obviously experience a decline in σs if the metal particle surface is oxidized. From this it follows that σs can be lowered by raising the temperature and/or oxygen concentration in the final slow oxidation treatment of the metal particles so as to increase the degree of particle oxidation and thereby decrease the σs of the particles as a whole. When this method is used to realize low σs, however, the advanced degree of oxidation results in an oxygen content exceeding 25 wt % in the case of particles with a major axis length of 20–80 nm. Moreover, by this method of oxidizing the particle surfaces, the axial ratio of the metal core decreases with increasing oxidation so that the magnetic shape anisotropy decreases. This lowers Hc, degrades SFD and otherwise markedly degrades the magnetic properties. Excessive slow oxidation treatment of the particles is therefore not preferable in producing magnetic powder with high Hc. Owing these circumstances, magnetic particles having a major axis length of 20–80 nm that exhibit high Hc have not been obtainable when utilizing increased oxygen content of greater than 25% to lower σs.
Particle σs can also be lowered by decreasing the reducing temperature to obtain the metallic iron so as to reduce the particle reduction rate. However, this suppresses growth of crystal grains inside the particles and therefore increases the grain boundary ratio, causes meso-pores, micro-pores and the like to remain, and produces magnetic poles by increasing the irregularity of the particle surfaces. The result is marked degradation of magnetic properties. Excessive reduction of the particle reduction rate is therefore undesirable in producing magnetic powder with high Hc. Owing to the different points explained in the foregoing, there is a need for development of a technology capable of lowering σs without lowering Hc.
Despite earlier research efforts such as those discussed above, no iron-base magnetic powder suitable for the high-sensitivity read MR heads (AMR, GMR and TMR heads) indispensable for tomorrow's high recording density systems can be said to have become available up to now.
The need felt for economical, industrial-scale production of magnetic powders meeting the aforesaid requirements remains far from met. In the field of iron-base magnetic powders that utilize acicular (including needle-like, spindle-like and flat-needle-like) iron oxy-hydroxide and iron oxide as starting material, for example, there has not yet been realized a magnetic powder suitable for high recording density systems using high-sensitivity read MR heads, namely, a magnetic powder that simultaneously satisfies the properties of a major axis length of not greater than 80 nm and σs of not greater than 90 emu/g while still achieving an Hc of 2,200 Oe or greater. The object of the present invention is to provide a magnetic powder of this description.